Security is a serious problem in all types of buildings, e.g. commercial, industrial, office, residential, parking garages, transportation facilities, hotels, hospitals, and others. Prompt detection and location of an intrusion, threat or unauthorized occupant is crucial. Traditional security systems are arranged to go into alarm if any one or more detectors are activated. They are thus of very limited value during periods of occupancy when the sensors are likely to be activated by legitimate occupants. Moreover, once alarmed these systems provided little, if any, further information about changes in the situation which caused the alarm. Many such security systems are simply turned off when the monitored areas must be occupied by legitimate persons.
A more useful security system would be one in which monitored areas could be freely used by legitimate occupants at any time, but which would quickly recognize suspicious or non-authorized presence or activity and provide precise location of the threatening occupancy before and after an alarm is created. Such a system would ideally be passive, that is it would require no special keys, cards, combinations, or other overt actions on behalf of legitimate occupants.
Stairwells are particularly notorious areas for staging crimes against persons and property: theft, burglary, robbery, rape, mugging and the like. Generally, facilities are easily entered during the day. An intruder can hide in a stairwell until activity subsides and then enter the building and move freely about. The relative privacy and confinement of stairwells are particularly attractive to muggers and rapists.
Unfortunately, fire and safey codes often conflict with good security practices: one-way doors which allow only entry into stairwells on upper floors and exit at only the street floor are increasingly prohibited by local building codes.
Conventional security systems often use only magnetic switches on each door of each stairwell in a building. In a typical forty story building with four stairwells, a guard at a central control console must monitor 160 door switches. A typical switch signals a guard who notes the time on a computerized logging system. If the doors are one-way only, they cannot be used by those legitimately traveling between floors without using the elevator. There is no information provided as to whether someone is in the stairwell and, if he is, whether or not his occupancy is suspicious or normal.